Spectrophone assembly for identifying and detecting multiple species

ABSTRACT

A spectrophone assembly comprises a single detector chamber, a plurality of lasers, a gas inlet for supplying a gas sample to the single detector chamber, and at least one microphone. The detector chamber has an internal geometry arranged to be simultaneously acoustically resonant at a plurality of different resonant frequencies. Each laser operates at a different wavelength and is positioned to emit radiation into the single detector chamber, and is operable to emit radiation that is amplitude modulated at a frequency rate corresponding to a particular resonant frequency different from the resonant frequency of each other laser, simultaneously with each other laser. The microphone(s) are positioned in the single detector chamber so that each microphone is located at or near a maximum of a corresponding acoustic resonance defined by the internal geometry of the detector chamber.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/797,375 filed on May 3, 2007, which is a continuation of U.S. patentapplication Ser. No. 11/086,280 filed on Mar. 23, 2005, now abandoned.

FIELD OF THE INVENTION

This invention relates to the identification and determination of theconcentrations of multiple species in a gas sample by means of aspectrophone.

BACKGROUND OF INVENTION

In laser based photo acoustic spectroscopy, each molecular specie in adetection chamber is basically detected from the response toillumination by laser radiation of a specific wavelength. Absorption ofsuch radiation by a specie in the detector chamber at the specificwavelength produces an amplitude modulated pressure which is detected bya microphone in the detector chamber. Generally, if more than one specieis involved, with interference from unwanted species is to be taken intoaccount, then operation at corresponding different wavelengths isrequired. The procedure involved ultimately sorts out different speciesand/or interfering components.

Such a procedure normally requires the operation of a spectrophone atdifferent wavelengths in time sequence, that is to say requires that thelaser be tuned in time ordered sequence to different wavelengths. Wheneach wavelength arises from a different and separate source, such as aset of semiconductor lasers each operating at a different wavelength,the illumination from each such laser is injected into the spectrophonein timed sequence. Thus, measurement of multiple species cannot becarried out simultaneously and consequently requires more time for thespecies to be identified and their concentrations determined.

Further information in this respect can be found in the followingreferences:

-   Kreuzer, L. B., Journal of Applied Physics, 42, 2934 (1971).-   Rosengren, L-G., Infrared Physics, 13, 173 (1973).-   Minguzzi, P., Tonelli, M., and Carrozzi, A., Journal of Optical    Spectroscopy, 96, 294 (1982).-   Morse, P. M., “Vibration and Sound” (McGraw-Hill, New York, 1968).-   West, G. A., Barrett, J. J., and Siebert, D. R., Review of    Scientific Instruments, 54, 797 (1983).

It is therefore an object of this invention to provide a methodsimultaneously identifying and determining the concentrations ofmultiple species by means of a spectrophone.

SUMMARY OF INVENTION

In one aspect, the present invention is directed to a spectrophoneassembly. The spectrophone assembly comprises a single detector chamber,a plurality of lasers, a gas inlet for supplying a gas sample to thesingle detector chamber, and at least one microphone. The detectorchamber has an internal geometry arranged to be simultaneouslyacoustically resonant at a plurality of different resonant frequencies.Each laser operates at a different wavelength and is positioned to emitradiation into the single detector chamber, and is operable to emitradiation that is amplitude modulated at a frequency rate correspondingto a particular resonant frequency different from the resonant frequencyof each other laser, simultaneously with each other laser. The at leastone microphone is positioned in the single detector chamber so that eachmicrophone is located at or near a maximum of a corresponding acousticresonance defined by the internal geometry of the detector chamber.

In one embodiment, the internal geometry of the detector chambercomprises at least two cylindrical tubes internally connected to oneanother.

In one embodiment, the spectrophone assembly further comprises at leastone power meter, a filter, a plurality of phase-lock amplifiers, and acomputer. The at least one power meter is sensitive to the frequencyrate and wavelength of each laser and is positioned to receive radiationfrom the lasers after said radiation has passed through the detectorchamber. The filter is coupled to the at least one power meter andoperable to separate exit power measured by the at least one power meterinto power components for each of the frequency rates. Each phase-lockamplifier is coupled to a corresponding microphone to receive a signaltherefrom, referenced to the frequency rate corresponding to theposition of its respective microphone, and operable to evaluate themicrophone signals and convert them into direct current values. Thecomputer is coupled to the filter to receive the power componentstherefrom and to the phase-lock amplifiers to receive the direct currentvalues therefrom, and is operable to use the power components and thedirect current values to generate specie identifications. The assemblymay include a single power meter sensitive to the frequency rate and thewavelength of each laser, or a plurality of power meters, each powermeter being sensitive to the frequency rate and the wavelength of aparticular laser.

DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example,with reference to the accompanying drawings, of which:

FIG. 1 is a schematic view of a spectrophone assembly in accordance withone embodiment of the invention;

FIG. 2 is a graph showing the separation between modulation frequenciesω₁ and ω₂;

FIG. 3 a is a side view of a spectrophone chamber in accordance withanother embodiment of the invention;

FIG. 3 b is an end view of the spectrophone chamber of FIG. 3 a;

FIG. 4 a is a side view of a spectrophone chamber in accordance with astill further embodiment of the invention;

FIG. 4 b is an end view of the spectrophone chamber of FIG. 4 a;

FIG. 5 a is a side view of a spectrophone chamber in accordance with yetanother embodiment of the invention; and

FIG. 5 b is a side view of the spectrophone chamber of FIG. 5 a.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring first to FIG. 1 of the drawings, a spectrophone assemblyincludes a cell or chamber 3 which in use is illuminated by two lasers1, 2 with different wavelengths λ₁, λ₂ respectively. The chamber 3 isformed by two cylindrical tubes with internal length and diameter L₁, D₁and L₂, D₂ respectively, the two tubes being internally connected andvacuum closed to the outside.

The lasers 1, 2 are power modulated at frequencies ω₁ and ω₂ bymodulators 1 a and 2 a respectively and, as references, thesefrequencies are sent to separate phase-lock amplifiers 10, 11. The laserbeams are guided into the chamber 3 by fiber optics or wave guides 4 andthe two beams are combined by a dichroic mirror 5 which transmits thebeam from laser 1 and reflects the beam from laser 2.

The chamber 3 is closed by two radiation transmitting windows 6 throughwhich the laser beams pass before being received by a radiation powermeter 7 adjacent the exit window 6. A single power meter 7 is used tomeasure the exit powers for both power beams simultaneously. Thus, thepower meter 7 must have a response time fast enough to detect beams atboth modulation rates ω₁ and ω₂. Alternatively, separate power meterseach sensitive to a corresponding modulation rate may be used.

The exit power P measured by power meter 7 is separated by a filter 8into power components for ω₁ and ω₂ which are then sent to a computer 14for signal normalization purposes. A gas sample containing trace amountsof the species of interest is passed into and out of the chamber 3through valved ports 9. The gas sample is typically air at a pressure ofabout one atmosphere. The sample may be a static gas fill or may becontinuously flowed through the chamber 3. The necessary electric powersupplies are of course provided as will be readily apparent to a personskilled in the art.

The chamber 3 is provided with microphones M₁, M₂. The acousticresponses chosen for operation should be sufficiently separated infrequency space such that there will be no overlap in the response fromthe microphones. This can be effected by proper design and selection ofthe internal geometry of the chamber 3. FIG. 2 illustrates a typicalfrequency separation of the two acoustic resonances in this embodiment.Such frequency separation provides a basic filter between the responsesof the microphones and also filters substantially all acoustic noiseand/or erroneous signals arising from outside the bandwidths of thesubject resonances.

Referring now again to FIG. 1, if the specie to be detected has anabsorptivity at λ₁, the absorption will produce gas heating which,because of the fixed chamber volume, causes a pressure change modulatedat a frequency of ω₁ which is sensed by internal microphone M₁. Theresulting electronic signal e_(s1), from microphone M₁ is fed to andmeasured by lock-in amplifier 10. The modulation rate ω₁ corresponds toan acoustic resonance frequency at ω₁ which amplifies the pressurechanges at this modulation rate. The frequency bandwidth of theresonance is sufficiently narrow to effectively prevent frequencyoverlap, within its bandwidth, with other resonances and thereby filtersout acoustic signals from any source at frequencies outside thebandwidth of the resonance at ω₁. The resonance frequency is determinedby the internal geometry of the chamber 3. Microphone M₁ is located ator near a maximum of the pressure standing wave 12, the amplitude ofwhich is shown in FIG. 1.

In a similar manner, microphone M₂ is located near a maximum of pressurestanding wave 13 in the side arm with length and diameter L₂, D₂. Theresonance in this case is at frequency ω₂ and outside the bandwidth ofthe resonance at ω₁. The signals e_(s1) and e_(s2) from microphones M₁and M₂ respectively are fed for processing to separate phase-locked(lock-in) amplifiers 10, 11, each referenced to the correspondingfrequencies ω₁ and ω₂ respectively. The phase-locked amplifiers evaluatethe microphone signals, convert them into direct current values andsubsequently feed them into the computer 14. The exit powers of the twobeams P(ω₁) and P(ω₂) required for normalization of the microphonesignals are also fed into the computer 14. The computer analyzes thecomputer data and produces the specie identifications and theirconcentrations for display.

The acoustic resonances of the chamber 3 are defined by its internalgeometry in accordance with the following equation:

ω_(kmn) =πc[(k/L)²+(β_(mn) /R)²]^(1/2)  (1)

where ω_(kmn) is the acoustic resonance frequency, in radians persecond, defined by a cylindrical section of length L between the endboundaries and of internal radius R, c is the velocity of sound for thegas at the pressure and temperature inside the chamber 3, k is aninteger having values corresponding to longitudinal harmonics, andβ_(mn) is the n^(th) root of the derivative of the Bessel functionJ_(m)(πβ), of order m, with respect to β.

It should be noted that the acoustic resonance is a pressure standingwave where the boundaries defining the length L can be any discontinuityin the cross section, such as the window boundary at each end of thechamber 3. The windows need not even be present, i.e. the ends of thecell may be open. The basic fact is that these boundaries define thestanding wave nodes of zero pressure. Each tubular section of the cell(as shown being utilized by microphones M₁ and M₂) has available to it anumber of resonances defined by the values of k, m and n and the sum anddifference resonance frequencies by various combinations of resonancesarising from the two sections shown in FIG. 1. All of the differingresonances can be used to increase the number of radiation sources ofdifferent wavelengths illuminating the chamber 3 where each source ismodulated and its microphone response is processed at its resonancefrequency.

There are many internal geometrical configurations of the chamber 3which promote acoustic resonances. To illustrate the principlesinvolved, FIGS. 3 a, 3 b, 4 a, 4 b, 5 a and 5 b show schematics of someconfigurations (with the pressure profile indicated by dotted lines) forsome resonant pressure standing waves. The maxima are the most desirablelocations for a microphone.

FIGS. 3 a and 3 b show fundamental resonance modes available with thegeometry illustrated. The expansion bulbs 16, 17, 18 define the lengthsto determine the resonance frequency by equation (1) for each case. Theexpansion bulbs 16, 17, 18 provide an abrupt change in the tube crosssection which is sufficient to force a pressure node (zero pressurepoint) within the vicinity of the entrance to the bulb. The chamber 15need not be closed at both ends. As shown, one end is open. FIG. 3 showsthe best locations X for a microphone for each resonance. A singlemicrophone of sufficient frequency bandwidth can be located in anoverlapping region of two or more resonances where the pressure value ineach case is non-zero.

FIGS. 4 a and 4 b show a configuration with two resonance side arms 19,20 to provide difference resonance frequencies, the side arms 19, 20having non-equal length and/or diameters as illustrated. Various otherresonances, all of different frequencies, can of course be obtained fromthe geometry shown.

FIGS. 5 a and 5 b show a configuration similar to that of FIGS. 4 a and4 b except that the side arm tubes 19, 20 are replaced by the geometriesof disks 21, 22 of radii R₁, R₂ respectively. The acoustic resonances inthese disks will be dominated by the second term (π c β_(mn)/R, i.e. theradial patterns of standing pressure waves, of equation (1). Additionalresonance frequencies are also obtainable from harmonics or overtonesand combinations in terms of sums and difference frequencies of thesefundamental resonances. Further, the same principles apply torectangular cross sections or any geometry where there will be pointsbetween which pressure standing waves can be produced.

Thus, as described above, a photo acoustic cell can be simultaneouslyilluminated by a number of radiation sources, each of differentwavelength, and simultaneously analyzing a gas sample in the cell forits specie identifications and concentrations. The gas pressure in thechamber may be in the range of from about 0.1 TORR to as high aspractically possible, and the detectable concentration of a specie mayrange from a trace to 100%.

The advantages and other embodiments of the invention will now bereadily apparent to a person skilled in the art, the scope of theinvention being defined in the appended claims.

1. A spectrophone assembly, comprising: a single detector chamber; thedetector chamber having an internal geometry arranged to besimultaneously acoustically resonant at a plurality of differentresonant frequencies; a plurality of lasers each operating at adifferent wavelength and positioned to emit radiation into the singledetector chamber; each laser operable to emit radiation that isamplitude modulated at a frequency rate corresponding to a particularresonant frequency different from the resonant frequency of each otherlaser, simultaneously with each other laser; a gas inlet for supplying agas sample to the single detector chamber; and at least one microphonepositioned in the single detector chamber, each microphone located at ornear a maximum of a corresponding acoustic resonance defined by theinternal geometry of the detector chamber.
 2. The spectrophone assemblyof claim 1, wherein the internal geometry of the detector chambercomprises at least two cylindrical tubes internally connected to oneanother.
 3. The spectrophone assembly of claim 1, further comprising: atleast one power meter sensitive to the frequency rate and the wavelengthof each laser and positioned to receive radiation from the lasers aftersaid radiation has passed through the detector chamber; a filter coupledto the at least one power meter and operable to separate exit powermeasured by the at least one power meter into power components for eachof the frequency rates; a plurality of phase-lock amplifiers, eachphase-lock amplifier being: coupled to a corresponding microphone toreceive a signal therefrom; referenced to the frequency ratecorresponding to the position of its respective microphone; and operableto evaluate the microphone signals and convert them into direct currentvalues; a computer coupled to the filter to receive the power componentstherefrom and to the phase-lock amplifiers to receive the direct currentvalues therefrom, and operable to use the power components and thedirect current values to generate specie identifications.
 4. Thespectrophone assembly of claim 3, wherein the at least one power metercomprises a single power meter sensitive to the frequency rate and thewavelength of each laser.
 5. The spectrophone assembly of claim 3,wherein the at least one power meter comprises a plurality of powermeters, each power meter sensitive to the frequency rate and thewavelength of a particular laser.